Backspace without CTRL should send '^?'

Summary:
This matches XTerm behaviour, so that pressing Alt+Backspace will send
\E + Del, which is the expected behaviour in some apps (e.g. emacs),
and it also matches the Konsole behaviour before commit
5345bbb0afa8edd6ba4909eca2495745df36f6a1

BUG: 394276
FIXED-IN: 18.04.2

Test Plan:

• Make sure you have the default .keytab file without any modifications
• Run emacs -Q -nw, press Ctrl+h k, to invoke the describe key functionality
• Press Alt+Backspace, it'll send C-M-h
• Apply the diff and test again, now it should send M-Del

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D12899

(cherry picked from commit be7181c93ad0f36953555e2cee402e193da2d5a9)